This invention relates to the field of 3D displays, head-mounted displays and moving projection displays and is a way of increasing their field of view.
Projection displays conventionally comprise a two-dimensional (2D) array of light emitters and a projection lens. The lens forms an image of the array at some plane in space, and if this imaging plane is far from the projection lens then the effect of the projection lens is to collimate light from any pixel on the two-dimensional array.
It is also possible for a large-diameter projection display to be placed behind a liquid-crystal a display or some other spatial light modulator in order to synthesize a three-dimensional image, as described in Travis, A. R. L. , xe2x80x9cAutostereoscopic 3-D Display,xe2x80x9d Applied Optics, Vol. 29, No. 29, pp4341 to 4343, Oct. 10, 1990. One pixel at a time of the two-dimensional array of light emitters is illuminated, and an appropriate view of a three-dimensional object is thus simultaneously displayed on the liquid-crystal display in such a way that the view of the three-dimensional object is only visible if observed from the direction in which the rays of light collimated by the projection lens from the pixel are travelling. A sequence of views is repeated at a rate faster than that at which the eye can detect flicker, thereby time-multiplexing a three-dimensional image. It is furthermore possible in principle to create a holographic three-dimensional image by placing a two-dimensional array of point source light emitters in the focal plane of the projection lens, illuminating each point source in turn, and displaying appropriate holograms on a liquid-crystal display placed on top of the projection lens so that each hologram is made visible to a different point of view in turn.
Projection displays are most commonly directed so that the image of the array falls on a large translucent screen, and a viewer looking at the screen will see a greatly magnified image of the picture that is displayed on the two-dimensional array. However, it is becoming increasingly common for small projection displays to be mounted on the head of the viewer so that the projection display is directed towards the viewer""s eye, and light collimated by the projection lens from a single pixel on the two-dimensional array of light emitters is subsequently focused by the viewer""s cornea onto the viewer""s retina so that the viewer sees an apparently distant image often known as a virtual image.
Head-mounted displays are bulky and users would prefer them to be flat. A head-mounted display can be made flatter, for example, using a slab waveguide incorporating a weak hologram, as disclosed in Amitai, Y., Reinhorn, S. and Friesem, A. A., xe2x80x9cVisor-display design based on planar holographic optics,xe2x80x9d Applied Optics, Vol. 34, No. 8, pp. 1352 to 1356, Mar. 10, 1999. Light from a cathode-ray tube and a further hologram can be coupled into the waveguide, and this light will be diffracted out of the waveguide by the weak hologram in directions which are determined by the pixel within the cathode-ray tube from which the light was emitted.
Three-dimensional images synthesized as described above by time-multiplexing the illumination of a liquid-crystal display require the liquid-crystal display to have a fast-switching array of thin-film transistors and these are expensive. Trayner and Orr (U.S. Pat. No. 5,600,454) have demonstrated a device which avoids this by placing a hologram behind a conventional liquid-crystal display that directs the illumination of alternate rows to a left-eye or right-eye view. But both this and the switched illumination concept described above are bulky, and users would prefer that three-dimensional displays were flat.
Instead, a flat-panel three-dimensional display can be made by combining a projection display with a screen on which light shone parallel to the surface of the screen is ejected at one of a set of selectable lines along the screen, as disclosed in WO 98/15128. One line at a time on the screen is selected, and simultaneously the projection display projects a line of pixels parallel to the screen so that they are ejected at the selected line. The same line of pixels on the projection display is altered repeatedly as each of the series of lines on the screen is selected in turn in such a way as to time-multiplex a complete image on the screen only one line of the projection display is used, so the array of light emitters need be only one line high, and if the emitted light is collimated in the plane of the screen then the projection lens need be only one or two millimetres high so that the combined projector and screen are flat.
With this construction if it is light from a three-dimensional display, albeit one whose array of light emitters is only one pixel high, which is directed parallel to the surface of the screen of selectable lines, then the image formed on the screen is three-dimensional. The three-dimensional display might comprise an array of light emitters behind a projection lens with a liquid-crystal display in front of the projection lens as described above, but in order to put up several views within one line period of the display, the switching rate of the liquid crystals would need to equal the number of views times the line rate of the display, and few liquid-crystal mixtures switch this fast.
Many other kinds of autostereoscopic and holographic three-dimensional display concepts exist and any could possibly be adapted to be used in a flat-panel system. Particularly interesting is an old concept comprising a group of small video projectors in the focal plane of a field lensxe2x80x94see A. R. L. Travis, Proc. IEEE Vol. 85, no. Nov. 11, 1997, pp. 1817-1832. Each projector is positioned to form a view in the plane of the field lens just as if the lens were a translucent screen, but unlike a translucent screen the field lens collimates the light so that the picture is visible from only a single direction. The other projectors form views which are made visible by the field lens to other directions so that the viewer sees an autostereoscopic three-dimensional image.
The problem with this concept is that it is difficult to design a projection lens whose pupil equals the lens""s physical diameter; as a result there are gaps between the video projectors which form dark zones between adjacent views of the three-dimensional image. A slightly diffusive element can be used to reduce these gaps, but the angle of diffusion usually varies with incident light angle. Aberrations in the field lens mean that rays collimated by the lens from a single point hit the diffusion screen at slightly different angles of incidence over the diameter of the screen. This means that the angles of diffusion vary, and even though the variance is slight it is enough to cause observable gaps between the views nearer-normal (if the projector spacing is set to eliminate all overlap) or overlaps between views (if the projector spacing is set to eliminate gaps).
In fact, a further major problem with three-dimensional displays and head-mounted displays in particular is that their field of view is limited by the aberration of the projection lens. At fields of view beyond 20xc2x0 the lens collimates light so poorly that the image is too distorted for most applications.
The present invention aims to overcome or at least alleviate some of the problems with projection displays known in the art.
The present invention contemplates a wide-field-of-view projection display making use of a circularly symmetric lens, sometimes called a monocentric lens, and a corresponding curved array of light emitters; the centre of curvature of the array is at the centre of the circularly symmetric lens and the array is placed at or near the focal plane of the circularly symmetric lens. Circularly symmetric lenses have been used beforexe2x80x94see U.S. Pat. No. 5,132,839 (Travis), but they are difficult to manufacture in a bulk-optic design.
According to a first aspect of the invention therefore there is provided a wide field of view projection display comprising: a generally planar circularly symmetric lens and an array of light emitters, positioned along the focal circumference of the circularly symmetric lens so that light rays from each of the light emitters is substantially collimated by the lens in a different direction from its neighbouring light emitters; and a ray-diverting means for ejecting the collimated light out of the plane of the lens towards the viewer.
The ray-diverting means is generally in the shape of a panel co-planar with or parallel to the plane of the lens and preferably comprises line-selecting means for selecting one line at a time of a changing image from the array of light emitters so as to display that line. In this case, the ejection may be brought about by deflection of the rays from the panel at the selected line. For example, the panel may include a reflective sheet and a transducer for producing a localized, linear, acoustic or surface wave in the sheet, the presence of the wave at a given position causing reflection of the ray and thus the said deflection at that position. In this construction, the wave created by the transducer travels along the panel successively ejecting the lines in its path. Alternatively a rotating-mirror arrangement can be used to scan each line over the height of a screen.
Alternatively the diverting means may be a flat panel of material which is not opaque to the rays and which is preferably parallel to the lens and aligned with the lens edge through which the rays emerge, so that it acts as a slab waveguide.
The line-selecting means may be provided by a layer or strip on the panel or at any other position in the collimated beam of light, which is switchably reflective or transparent, the means for selecting the position at which the rays are ejected being adapted to change the state of the switchable layer. Such a layer may be a liquid-crystal display.
The switchable layer may work in transmission (so that the rays in the selected line travel through the layer and others are reflected) or in reflection (the selected line only is reflected. In this latter mode, the grating may be arranged to eject light only out of one surface of the panel in the direction of the switchable layer. Such a grating may be positioned within the panel. The selected line is then reflected back through the panel by the switchable layer. In this mode, the selecting means are therefore provided behind the panel, from the viewer""s perspective. Reference may be made to WO 95/15128, mentioned above, for more information.
Each light emitter used in the present invention may include a microdisplay, generally a small LCD with a collimated light source behind it, if transmissive, or in front of it, if reflective. The light emitter may simply be a microdisplay positioned at the focal circumference. Alternatively, each light emitter may comprise a microdisplay and an individual lens, arranged so that the microdisplay emits light which is then converged towards the individual lens. Each individual lens should be positioned on the focal circumference of the circularly symmetric lens and acts essentially as a point source from which collimated light can be produced.
In general the microdisplay will be one dimensional, consisting, of a row of columnar pixels, and the corresponding lens is cylindrical and separated from the microdisplay by its focal distance if the lens is cylindrical, the microdisplay is thus positioned on the focal circumference. Neighbouring microdisplays each project a single-line image of an object, the images differing only in the angle of view.
Alternatively, each light emitter may simply comprise a source of light positioned on the focal circumference of the circularly symmetric lens. In this construction, a one-dimensional switchable strip is provided in the path of the collimated rays. The strip is preferably between the circularly symmetric lens and the ray-diverting panel. If the light sources are point sources, the strip may be used to display a hologram by suitable addressing of the strip. Alternatively, abutting sources can be used to display an auto-stereoscopic view. The emitters are activated in turn and the pattern displayed on the strip is synchronized with the actuation of the emitters.
The display according to the invention may include a diffuser positioned in the collimated light to narrow gaps in the beam between the light from each light emitter. The diffuser may be formed as a diffraction grating or lenslet screen and is preferably positioned adjacent to the flat panel. The diffuser will generally be necessary for autostereoscopic displays in order to ensure that there are no gaps between views; holographic displays will not generally need a diffuser.
A frame store may be provided for each microdisplay to store successive images of a moving display before they are: applied to the projectors; this makes it possible to compensate for any optical deficiencies, or geometrical distortion such as shear due to the angle of projection of the side projectors.
In one preferred embodiment of the invention, a reflector, such as a mirror, is provided to at least one side of the panel to reflect an outer portion of the images from the more off-axis projectors that miss the panel, back onto the panel. Such a reflector reduces the gap that may be produced at the side of the image by rays from outer microdisplays in the array. A straightforward reflector would reflect the outer image portion across to the opposite of the image from its correct position. Preferably therefore, image-processing means are provided, to ensure that the reflected, pixels are reflected onto the correct side of the image. These may be provided in conjunction with the frame store and act to swap pixels at the outer edges of the frame store.
The display according to the invention may be arranged with the circularly symmetric lens, panel and microdisplay (where applicable) in substantially the same plane. Alternatively, the planes in which the panel and lens are formed may be adjacent and parallel. In this case, folding means are required to fold the optical system so that rays emitted from the edge of the lens are directed onto the panel. The folding means may also fulfill the function of retrieving rays which will otherwise miss the panel. The folding means may comprise a retroreflector, preferably situated next to the portion of the lens from which the rays emerge, and angled mirrors to either side of the retroreflector. The retroreflector is preferably positioned in a plane substantially perpendicular to the side mirrors and the prisms of the retroreflector run perpendicular to its longitudinal axis.
In another embodiment, for a virtual display, the rays at any position on the panel are ejected. To this end, the panel may include a weak diffraction grating, which causes collimated light to travel in a particular direction. The grating should be provided on the side of the panel through which the rays are ejected.